常用的神经网络结构和pytorch实现(二)

Resnet系列

关于resnet方面的基础介绍直接看这篇博客，不再叙述

https://www.cnblogs.com/henuliulei/p/13564468.html

resnet34的样子如下面

 resnet50，101等和34不同的点在于使用了

更深的瓶颈结构:

 

 

作者探索的更深的网络。 考虑到时间花费，将原来的building block(残差学习结构)改为瓶颈结构，如上图。首端和末端的1x1卷积用来削减和恢复维度，相比于原本结构，只有中间3x3成为瓶颈部分。

这两种结构的时间复杂度相似。此时投影法映射带来的参数成为不可忽略的部分（以为输入维度的增大），所以要使用zero padding的同等映射。
替换原本ResNet的残差学习结构，同时也可以增加结构的数量，网络深度得以增加。生成了ResNet-50，ResNet-101，ResNet-152. 随着深度增加，因为解决了退化问题，性能不断提升。
作者最后在Cifar-10上尝试了1202层的网络，结果在训练误差上与一个较浅的110层的相近，但是测试误差要比110层大1.5%。作者认为是采用了太深的网络，发生了过拟合。

常用的网络模型的结构图如下，其中ResNet-50拥有2500万参数

 resnet34的网络模型为

#coding:utf8
from .BasicModule import BasicModule
from torch import nn
from torch.nn import functional as F

class ResidualBlock(nn.Module):
    '''
    实现子module: Residual Block
    '''
    def __init__(self, inchannel, outchannel, stride=1, shortcut=None):
        super(ResidualBlock, self).__init__()
        self.left = nn.Sequential(
                nn.Conv2d(inchannel, outchannel, 3, stride, 1, bias=False),
                nn.BatchNorm2d(outchannel),
                nn.ReLU(inplace=True),
                nn.Conv2d(outchannel, outchannel, 3, 1, 1, bias=False),
                nn.BatchNorm2d(outchannel) )
        self.right = shortcut

    def forward(self, x):
        out = self.left(x)
        residual = x if self.right is None else self.right(x)
        out += residual
        return F.relu(out)

class ResNet34(BasicModule):
    '''
    实现主module：ResNet34
    ResNet34包含多个layer，每个layer又包含多个Residual block
    用子module来实现Residual block，用_make_layer函数来实现layer
    '''
    def __init__(self, num_classes=2):
        super(ResNet34, self).__init__()
        self.model_name = 'resnet34'

        # 前几层: 图像转换
        self.pre = nn.Sequential(
                nn.Conv2d(3, 64, 7, 2, 3, bias=False),
                nn.BatchNorm2d(64),
                nn.ReLU(inplace=True),
                nn.MaxPool2d(3, 2, 1))
        
        # 重复的layer，分别有3，4，6，3个residual block
        self.layer1 = self._make_layer( 64, 128, 3)
        self.layer2 = self._make_layer( 128, 256, 4, stride=2)
        self.layer3 = self._make_layer( 256, 512, 6, stride=2)
        self.layer4 = self._make_layer( 512, 512, 3, stride=2)

        #分类用的全连接
        self.fc = nn.Linear(512, num_classes)
    
    def _make_layer(self,  inchannel, outchannel, block_num, stride=1):
        '''
        构建layer,包含多个residual block
        '''
        shortcut = nn.Sequential(
                nn.Conv2d(inchannel,outchannel,1,stride, bias=False),
                nn.BatchNorm2d(outchannel))
        
        layers = []
        layers.append(ResidualBlock(inchannel, outchannel, stride, shortcut))
        
        for i in range(1, block_num):
            layers.append(ResidualBlock(outchannel, outchannel))
        return nn.Sequential(*layers)
        
    def forward(self, x):
        x = self.pre(x)
        
        x = self.layer1(x)
        x = self.layer2(x)
        print(x)
        x = self.layer3(x)
        x = self.layer4(x)

        x = F.avg_pool2d(x, 7)
        x = x.view(x.size(0), -1)
        return self.fc(x)

resnet50模型为

# coding:utf8
from .BasicModule import BasicModule
from torch import nn
from torch.nn import functional as F


class ResidualBlock(nn.Module):
    '''
    实现子module: Residual Block
    '''

    def __init__(self, inchannel, outchannel, stride=1, shortcut=None):
        super(ResidualBlock, self).__init__()
        self.left = nn.Sequential(
            nn.Conv2d(inchannel, int(outchannel/4), 1,1,bias=False),
            nn.BatchNorm2d(int(outchannel/4)),
            nn.ReLU(inplace= True),
            nn.Conv2d(int(outchannel/4), int(outchannel/4), 3, stride, 1, bias=False),
            nn.BatchNorm2d(int(outchannel/4)),
            nn.ReLU(inplace=True),
            nn.Conv2d(int(outchannel/4), int(outchannel), 1, 1,bias=False),
            nn.BatchNorm2d(outchannel))
        self.right = shortcut
        self.inchannel = inchannel

    def forward(self, x):
        out = self.left(x)
        residual = x if self.right is None else self.right(x)
        print(self.inchannel,out.shape,residual.shape)
        out += residual
        return F.relu(out)


class ResNet50(BasicModule):
    '''
    实现主module：ResNet34
    ResNet34包含多个layer，每个layer又包含多个Residual block
    用子module来实现Residual block，用_make_layer函数来实现layer
    '''

    def __init__(self, num_classes=2):
        super(ResNet50, self).__init__()
        self.model_name = 'resnet34'

        # 前几层: 图像转换
        self.pre = nn.Sequential(
            nn.Conv2d(3, 64, 7, 2, 3, bias=False),
            nn.BatchNorm2d(64),
            nn.ReLU(inplace=True),
            nn.MaxPool2d(3, 2, 1))

        # 重复的layer，分别有3，4，6，3个residual block
        self.layer1 = self._make_layer(64, 256, 3)
        self.layer2 = self._make_layer(256, 512, 4, stride=2)
        self.layer3 = self._make_layer(512, 1024, 6, stride=2)
        self.layer4 = self._make_layer(1024, 2048, 3, stride=2)

        # 分类用的全连接
        self.fc = nn.Linear(2048, num_classes)

    def _make_layer(self, inchannel, outchannel, block_num, stride=1):
        '''
        构建layer,包含多个residual block
        '''
        shortcut = nn.Sequential(
            nn.Conv2d(inchannel, outchannel, 1, stride, bias=False),
            nn.BatchNorm2d(outchannel))

        layers = []
        layers.append(ResidualBlock(inchannel, outchannel, stride, shortcut))

        for i in range(1, block_num):
            layers.append(ResidualBlock(outchannel, outchannel))
        return nn.Sequential(*layers)

    def forward(self, x):
        x = self.pre(x)

        x = self.layer1(x)
        x = self.layer2(x)
        x = self.layer3(x)
        x = self.layer4(x)

        x = F.avg_pool2d(x, 7)
        x = x.view(x.size(0), -1)
        return self.fc(x)

MobileNet v1